1. Field of the Invention
The present invention relates to a DC/DC converter.
2. Description of the Related Art
Various kinds of home appliances such as TVs, refrigerators etc., each receive externally applied commercial AC electric power for its operation. Also, electronic devices such as laptop computers, cellular phone terminals, and tablet devices are each configured to operate using commercial AC electric power, and/or to be capable of charging a built-in battery using the commercial AC electric power. Such home appliances and electronic devices (which will collectively be referred to as “electronic devices” hereafter) each include a built-in power supply apparatus (converter) configured to convert the commercial AC voltage to DC voltage. Alternatively, such a converter is built into an external power supply adapter (AC adapter) for such an electronic device.
FIG. 1 is a block diagram showing an AC/DC converter 100r investigated by the present inventor. The AC/DC converter 100r mainly includes a filter 102, a rectifier circuit 104, a smoothing capacitor 106, and a DC/DC converter 200r. 
The commercial AC voltage VAC is input to the filter 102 via an unshown fuse and an unshown input capacitor. The filter 102 removes noise included in the commercial AC voltage VAC. The rectifier circuit 104 is configured as a diode bridge circuit which full-wave rectifies the commercial AC voltage VAC. The output voltage of the rectifier circuit 104 is smoothed by the smoothing capacitor 106, thereby generating stabilized DC voltage VIN.
The insulation-type DC/DC converter 200r receives the DC voltage VIN at its input terminal P1, steps down the DC voltage VIN and supplies an output voltage VOUT stabilized to the target value to a load (not shown) connected to its output terminal P2.
The DC/DC converter 200r includes a primary controller 202, a photo coupler 204, a shunt regulator 206, an output circuit 210, and additional circuit components. The output circuit 210 includes a transformer T1, a diode D1, an output capacitor C1, and a switching transistor M1. The output circuit 210 has a conventional topology, and accordingly, detailed description thereof will be omitted.
With the switching operation of the switching transistor M1, the input voltage VIN is stepped down, and the output voltage VOUT is generated. The controller 202 adjusts the duty ratio of the switching operation of the switching transistor M1 so as to stabilize the output voltage VOUT to the target value.
The output voltage VOUT of the DC/DC converter 200r is divided by the resistors R1 and R2. The shunt regulator 206 amplifies an error between the divided voltage (voltage detection signal) Vs and a predetermined reference voltage VREF, and draws (sinks) an error current IERR according to the error from the light emitting device (LED) of the photo coupler 204.
Accordingly, a feedback current IFB flows through the light receiving device (photo transistor) of the photo coupler 204, which corresponds to the error current IERR at the secondary side. The feedback current IFB is smoothed by a resistor and a capacitor, and is supplied to a feedback (FB) terminal of the controller 202. The controller 202 adjusts the duty cycle of the switching transistor M1 according to a feedback voltage VFB at the FB terminal.
The present inventors have investigated the AC/DC converter 100r, and have come to recognize the following problem. For a stable operation of the shunt regulator 206, a cathode current IK should be kept to some degree. The cathode current IK includes a component which contributes to the feedback operation and a constant bias component (i.e. idle current). For example, most of the shunt regulators on the market require the idle current of a several hundred microamperes (for instance, 700 microamperes), and this lowers an efficiency of the AC/DC converter 100r. 